As of now, logging into the 1.14 MP server will require an activated forum account - this only means that you have clicked the link in the verification email, there is no requirement to actually post or "be active" on the forums. This has been implemented in part so that the forum's additional moderation functionality will be usable for enforcing moderation actions on the MP server, and relatedly, to have this functionality available in case the Steam release results in a significant increase in the number of new players.

Regarding alt accounts and multiple accounts:
This does mean that any users with alt accounts or multiple accounts will need to register those accounts here on the forums as well, however there is no limitation on how many accounts can use the same email+password.

99 little bugs in the code, 99 little bugs
take one down, patch it around
-2,147,483,648 little bugs in the code

A registration button and quick explanation should be added ingame when you try to join the official servers. For newcomers of Steam it will be confusing to read that one is not registered, but doesn't know where he shall register.

Also registering in a forum for online play isn't user-friendly, so rather keep all the registration process ingame instead of linking to the forums. After or while registrating ingame you still can inform the user that the created account is at the same time the forum account and link to the forums.

Question: Since you can only use your registered account name in MP, what do you do with long-time inactive names? Will they be forever unavailable? Or is it intended to add options to use different player names while the forum name is just for login?
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Probably a bug: I made my password be autosaved, but when I left the lobby by pressing ESC and tried to rejoin, it again asked for my password. When I then logged out through the logout button, this time it saved the password (and now all is good).

I have to agree with tarik, I think it's a rather obtuse way of creating an account and turns away new players from multiplayer. It doesn't require a lot of effort, but basic presentation and getting into the game quickly is important for the audience in general to be engaged in the game.

Yeah, this was a decision made closer to the release of 1.14 than was ideal. This is something that's come up somewhat frequently however, so to quote an announcement from Discord:

We've been receiving a lot of feedback over the past day expressing confusion over the forum account requirement for the official MP server in 1.14. To clarify, this is indeed a necessary change and we intend to stay the course. However, we failed to make the details such as where one could go to register abundantly clear or easily accessible, and we'd like to apologize for that. We'll be rolling out a hotfix within the next few days aimed to make the process less confusing.

Allowing registration to take place entirely in-game is not something that we can add quickly, unfortunately, but my understanding is that there will at least be additional wording added along with a link to the forums so it's clearer what people need to do.

Question: Since you can only use your registered account name in MP, what do you do with long-time inactive names? Will they be forever unavailable? Or is it intended to add options to use different player names while the forum name is just for login?

It is always a possibility that inactive users decide to come back, so registered usernames are not removed. There are also no plans to add the ability for people to create other aliases after logging in.

Probably a bug: I made my password be autosaved, but when I left the lobby by pressing ESC and tried to rejoin, it again asked for my password. When I then logged out through the logout button, this time it saved the password (and now all is good).

Is this something you are able to reproduce consistently? I tried, and it correctly saved my password.

99 little bugs in the code, 99 little bugs
take one down, patch it around
-2,147,483,648 little bugs in the code

I suspect it has something to do with whether you are logged into wesnoth forum or not
It was happening all the time until I posted something on forum. After that no matter what it remembers my password.

There will most likely be Steam Overlay support at some point(and it's already working on some Linux distributions, somehow), though it may not be in 1.14. My understanding is that Steam Overlay somehow hooks into OpenGL/DirectX, and 1.14 still uses software rendering. One of the main goals of 1.15 though is to start using OpenGL, so at the very least it sounds like Wesnoth would get it for free as part of that transition.

99 little bugs in the code, 99 little bugs
take one down, patch it around
-2,147,483,648 little bugs in the code

Please could Server2 allow Wesnoth 1.14.* to connect? It currently allows only "1.13*dev,test*".

This would be useful for add-on authors wanting to test with multiple clients; creating multiple accounts makes it harder to use useful-for-testing names ("octa-addon_installed", "octa-_addon_not_installed", "octa-lang_fr", etc).

Please could Server2 allow Wesnoth 1.14.* to connect? It currently allows only "1.13*dev,test*".

This would be useful for add-on authors wanting to test with multiple clients; creating multiple accounts makes it harder to use useful-for-testing names ("octa-addon_installed", "octa-_addon_not_installed", "octa-lang_fr", etc).

You don't need an external server in order to test add-ons. Create a lan server(host networked game), then connect to your local ip address with the other clients
(connect to server).

Over the last ten years I have witnessed consistently fun and good natured behavior from the wesnoth multiplayer community and nothing that even remotely qualified for moderation.

What is the rationale for this sudden and significant shift towards so much security and verification and why is it now considered "necessary"? What happened that precipitated such a strong response as this?

I'm not going to go into details of any moderation actions that've been taken, but suffice it to say that such actions have indeed been necessary. I also wouldn't consider a login requirement to be "so much security and verification".

99 little bugs in the code, 99 little bugs
take one down, patch it around
-2,147,483,648 little bugs in the code